Conventionally, as a method of manufacturing a color selection mechanism for use in cathode ray tubes, JP 7(1995)-211229 A, for example, describes a method of welding a color selecting electrode to a frame, where the color selecting electrode includes an aperture grill provided with a plurality of parallel and vertical slits formed in a metal sheet. FIGS. 7A and 7B show a welding route during the welding process.
The color selecting electrode including the aperture grill is welded only to long frame sides 1 of a frame 3 made up of the long frame sides 1 and short frame sides 2. The welding route includes the following steps as shown in FIGS. 7A and 7B: a welding head moves down onto a center portion of the long frame side 1 (Step I), welding is conducted from the center portion toward one end of the long frame side 1 (Step II) followed by a step where the welding head moves up once (Step III), the welding head moves horizontally toward a center portion of the long frame side 1 (Step IV) followed by a step where the welding head moves down onto the long frame side 1 (Step V), welding is conducted from the center portion toward the other end of the long frame side 1 (Step VI), and the welding head retracts (Step VII) so as to complete the welding of the color selecting electrode to the frame 3.
However, in the case of applying this welding method to a tension-type shadow mask as described in JP 2000-77007 A where a tensile force parallel to a short side direction of a frame is applied to a shadow mask (color selecting electrode) and a tensile distribution is required for a long side direction of the frame so that a tensile force at a center portion is larger than a tensile force at an end portion, the following problem would occur: that is, the welding of the color selecting electrode to the frame with the conventional welding method described in JP 7-211229 A causes the concentration of a distortion in the color selecting electrode, which is generated due to welding heat, at a termination part of the welding route. Therefore so-called wrinkles are generated in the vicinity of the end portions of the long side direction of the color selecting electrode, which correspond to the termination of the welding route, thus deteriorating a picture quality of the cathode ray tube.